Below, the term “filament” designates a filamentary element that is not necessarily a monofilament. Thus, a filament may be a multifilament element, a tow, or a ply yarn. The filament may be made of one or more materials, such as a textile material or a metal, for example. The filament may optionally be treated.
Generally, in a strip for use in fabricating a tire, the filaments are spaced apart regularly from one another and they extend in directions that are coplanar and mutually parallel.
Document U.S. Pat. No. 4,274,821 discloses a device for extruding a rubber strip of this type.
That device includes means for channeling rubber to an extrusion orifice for forming the strip.
The channeling means comprise an upstream main channel connected to a rubber feeder device, together with first and second downstream secondary channels extending the upstream main channel and connected to the extrusion orifice via an extrusion chamber.
That device also has means for guiding the filaments, which means comprise filament guide channels having ends that are upstream and downstream relative to the direction in which the strip is extruded through the extrusion orifice. The downstream end opens out into the extrusion chamber.
The extrusion orifice presents a section that is substantially rectangular, being defined by first and second directions such that the dimensions of the orifice in said first and second directions determine respectively the thickness and the width of the strip. Furthermore, the outline of the extrusion orifice presents first and second orthogonal planes of symmetry that intersect at a line that is substantially parallel to the direction in which the strip is extruded through the extrusion orifice.
Each guide channel extends between its upstream and downstream ends by following a path that is substantially rectilinear and parallel to the first plane of symmetry so that the filaments penetrate into the extrusion chamber substantially parallel to the extrusion direction.
In the extrusion chamber, the filaments are embedded, being sandwiched between the rubber as channeled in the secondary channels opening out into the extrusion chamber.
Unfortunately, head losses are generated by the rubber rubbing against the walls of each secondary channel.
Since the main channel has the first plane as its plane of symmetry and since the two downstream secondary channels are substantially symmetrical to each other about the first plane, head losses in the two mutually symmetrical streams of rubber about the first plane and following respective different secondary channels are substantially equal in each secondary channel. As a result, the section of the strip has the first plane as its plane of symmetry.
Nevertheless, since each channel extends along a path that is curved between the main channel and the extrusion chamber, head losses differ depending on the path followed by the rubber in each channel. As a result the thickness of the strip is relatively irregular.